Since the inception of the modern pH scale, a variety of devices have been developed to monitor and interpret changes in the negative log of the concentration of hydrogen protons in a solution, “pH”.
For general or industrial applications, pH papers or liquid indicators that change color as the pH level of a solution varies are used. These indicators are convenient to use, but have limitations on their accuracy, and can be difficult to interpret correctly in some conditions.
For laboratory applications, a more accurate tool is employed that relies on electronic pH measurement means. This equipment typically consists of three parts: a pH measuring electrode, a reference electrode, and a high input impedance meter. The pH measuring electrode and reference electrode can be thought of as a battery, with a voltage that varies with pH of the measured solution. The system can be made up of a large glass bulb with a hydrogen ion sensitive coating. This coating creates a millivolt output that varies with changes in relative hydrogen ion concentration inside and outside of the bulb. The reference electrode can consist of a combination of metals, chemicals, and liquid commonly known as electrolytic fluid or gel, that create a millivolt output that does not vary with changes in hydrogen ion concentration.
In medical applications where the environmental pH of the esophagus or pharyngeal regions need to be measured, a smaller, more compact sensor system is utilized. The pH sensing element usually consists of an exposed antimony metal segment, that changes voltage with the change in pH, and a silver/silver chloride reference electrode, that does not change voltage with the change in pH. The reference electrode of these pH sensors is usually protected from outside contaminants by nesting the element within the body of the pH sensor and surrounding it with ion conducting electrolytic gel. The reference wick, typically a strand of fibrous material, is used as a conduit between the ion conducting gel and the environment which is to be measured.
As moisture is necessary to maintain the ion conducting properties of the electrolytic gel, these esophageal or pharyngeal pH sensors must be packaged in a way as to retain sufficient moisture for ion conduction. Manufacturing a pH sensor that has a fluid element, as well as packaging to retain that moisture during shipping and storage, poses a number of challenges such as moisture retention, fluid migration, and component deterioration.